The present invention relates to a method for manufacturing cold water infusing leaf tea. The method involves fermenting tannase pre-treated dhool (macerated tea leaves) under solid-state conditions in the presence of hydrogen peroxide. The dried leaf product infuses in cold water to give good flavour and colour.
Black leaf tea is traditionally produced by oxidising and drying freshly plucked green tea leaves. Tea, the beverage, is generally prepared in Commonwealth countries by brewing these tea leaves in freshly boiled water for a few minutes and adding milk, and perhaps a little sugar. However in some countries, notably the United States (or more accurately, parts thereof) tea is more commonly enjoyed as an iced beverage.
Such a beverage cannot be prepared conveniently by infusing traditionally manufactured tea leaves in cold water. Instead, Americans either infuse the leaves in hot water, remove the leaves and place the infusion in a refrigerator until it is ready to consume or place tea leaves in cold water in sunlight to infuse slowly over a period of hours.
The numerous compounds in the leaves that give the beverage its unique organoleptical properties are only sparingly soluble in cold water. A more convenient option that has become available in the 1970""s is the use of cold soluble tea-based powders.
There are numerous methods for making cold water soluble tea powders.
United States patent specification U.S. Pat. No. 4,051,264 (Lipton/Sanderson) discloses a method for making a cold-water soluble leaf tea extract. Tea leaves are pre-treated with tannase under anaerobic conditions to generate a cold-water infusing tea with good colour, yield and flavour.
United States patent specification U.S. Pat. No. 3,812,266 (Sanderson/Coggon) discloses a method that involves converting green tea to black using tannase and natural tea enzymes. The method also includes a tannase pre-treatment step, but in a slurry system, followed by oxidation by natural tea enzymes to convert green tea into black, and generate tea powders, which are both hot and cold water soluble. In some examples hydrogen peroxide is added, to xe2x80x9cshorten the processxe2x80x9d. The proposed mechanism for enhanced cold-water soluble colour generation resulting from tannase-treatment (elevated epitheaflavic acid levels) is now known to be incorrect, and no mechanism was presented to explain the effect of adding the hydrogen peroxide.
European patent specification EP 760,213 A1 (Unilever) discloses a method of enhancing colour in a tea-based foodstuff. The method involves using a tannase pre-treatment (on leaf or extract) followed by treatment with exogenous peroxidase and hydrogen peroxide to generate cold-soluble colour.
International patent publication WO 97/40699 (Unilever) concerns tea processing with zeolites to generate colour. There are examples of adding zeolite following tannase treatment to generate cold-water soluble tea.
United States patent specification U.S. Pat. No. 4,639,375 (PandG, Tsai) discloses treating black tea with tannase, together with other cell-wall digesting enzymes, to generate cold-water soluble instant tea powders.
Convenient as cold water soluble tea powders can be, for many consumers the quality of the final beverage is not equal to that prepared from hot infused leaves. Other consumers prefer not to use powders as they perceive them to be artificial and therefore xe2x80x9cunnaturalxe2x80x9d.
The present inventors have surprisingly found that it is possible to make a leaf tea that infuses in cold water to give a beverage with good colour and flavour that is as acceptable to consumers as a hot infused black leaf tea that has been refrigerated. Furthermore this product can be made by modifying the traditional black tea manufacturing process.
In broad terms the present invention relates to a method for preparing a cold water infusing leaf tea comprising the steps of macerating green tea leaves, treating the macerated leaves with tannase, fermenting the tannase-treated macerate in the presence of an amount of hydrogen peroxide that is sufficient to activate endogenous peroxidases, and drying the fermented leaf material to yield the cold water infusible leaf tea.
The invention also relates to a method for generating colour species in a cold water soluble tea product comprising adding hydrogen peroxide to a tannase-treated macerate of green tea in a quantity that is sufficient for the endogenous peroxidases to oxidise gallic acid liberated by the tannase treatment.
xe2x80x9cTeaxe2x80x9d for the purposes of the present invention means leaf material from Camellia sinensis var. sinensis or Camellia sinensis var. assamica. It also includes rooibos tea obtained from Aspalathus linearis however that is a poor source of endogenous fermenting enzymes. xe2x80x9cTeaxe2x80x9d is also intended to include the product of blending two or more of any of these teas.
xe2x80x9cLeaf teaxe2x80x9d for the purposes of this invention means a tea product that contains one or more tea origins in an uninfused form.
xe2x80x9cCold water solublexe2x80x9d for the purposes of this invention means giving good colour, flavour and mouthfeel in a short infusion time i.e. less than 10 minutes, but preferably less than 5 minutes at a temperature at or above 4xc2x0 C.
The macerated leaves are preferably tannase treated under anaerobic conditions. The process is effective without this anaerobic incubation provided sufficient tannase is used. The tannase treated macerate is preferably fermented under standard conditions to produce elevated levels of theaflavins and gallic acid prior to the addition of hydrogen peroxide.
For the avoidance of doubt the word xe2x80x9ccomprisingxe2x80x9d is intended to mean including but not necessarily xe2x80x9cconsisting ofxe2x80x9d or xe2x80x9ccomposed ofxe2x80x9d. In other words the listed steps or options need not be exhaustive.
Tea manufacture, especially black tea manufacture, traditionally comprises four basic steps: withering, rolling, fermenting and firing.
Withering is a process whereby the plucked tea leaves are stored for periods of time (perhaps up to 24 hours), during which they undergo various biochemical and physical changes which often includes a loss of moisture.
Maceration follows the withering step, and traditionally the withered leaves are optionally rolled to bruise and crush the leaves i.e. break down the plant tissue structure. This will have the effect of liberating fermentable substrates and fermenting enzymes from within the plant cells and tissue. Modern tea manufacture usually includes this step however the plant cells and tissue is broken down by passing tea, which has usually been withered, through a cutting machine.
The next step is commonly called fermentation but that is a misnomer. xe2x80x9cFermentationxe2x80x9d is commonly used in the context of brewing alcohol to describe the action of exogenous enzymes. However in the tea world it is used to refer to the oxidative process that tea undergoes when certain endogenous enzymes and substrates are brought together by mechanical disruption of the cells by tearing or cutting the leaves. During this process colourless catechins in the leaves are converted to a complex mixture of yellow and orange to dark-brown substances and producing a large number of aromatic volatile compounds.
The colourful oxidation products include theaflavins and thearubigens. Theaflavins comprise several well-defined catechin condensation products that are characterised by their benzotropolone ring. Thearubigens are a group of undefined molecules with a large variance in molecular weight. They have a large variety of colours ranging from yellow to dark red and brown.
The fermented product is fired and dried to give a black leaf tea. The firing involves heating and drying the tea to destroy the fermenting enzymes and thereby arrest fermentation. It results in a reduction of moisture content to below 5%, and also leads to further chemical oxidation and changes in tea aroma. This generally involves exposing the tea to a blast of hot, dry air in a dryer.
The present invention relates to method for making cold soluble black leaf tea. The present inventors have found that this can be achieved by modifying some of the steps of the traditional tea manufacturing process just described.
A preferred embodiment of the method of the invention is depicted in FIG. 1. In that preferred embodiment freshly plucked green tea leaves are withered in the normal way using any of the art known techniques. Withering is not essential to the invention but it can be a useful means to enhance tea aroma and also to reduce initial moisture content (which is important as moisture will be added with the tannase and peroxide, and drying efficiency can be affected at high moisture contents i.e. greater than 76%).
The leaves are macerated, which could mean being comminuted by a rotorvane and/or by a number of CTC (cut-tear-curl) machines.
In a first departure from traditional black tea manufacture the macerated leaves are treated with tannase (flavanol gallate esterase) to generate degallated catechins and gallic acid. This subsequently leads to the generation of theaflavins and non-gallated thearubigens during fermentation (which are more soluble than the gallated ones).
The general reaction catalysed by tannase is the cleavage of gallate ester linkages, both on gallated catechins and also from other gallated compounds within the leaf. Tannase is also well known to improve the clarity of tea products since galloyl groups are important in cream formation and tannase has been used extensively for the degallation and solubilisation of black tea cream.
Tannase is known to be useful for treating green tea prior to slurry fermentation. For example the aforementioned U.S. Pat. No. 3,812,266 (Sanderson et al) discloses using tannase to reduce the amount of tea cream in liquors. Improved colours generated by the process were also noted.
The present invention however does not require the tea to be suspended and fermented in a slurry. Indeed this would be counter-productive because the components necessary to give good colour and flavour from the leaf would be prematurely extracted into the slurry. Rather the tea is fermented under solid-state conditions. This is an important distinction. The inventors previously thought that extraction of the catechins into a slurry liquor was essential for efficient action of tannase. They were surprised that direct application of tannase to the dhool in a solid-state led to efficient (i.e. almost complete) catechin degallation and high levels of theaflavin generation. They were even more surprised that the leaf product infused in cold water.
Tannase treatment degallates the gallated catechins ECG and EGCG to produce the degallated catechins EC and EGC. On subsequent oxidation during fermentation the catechins EC and EGC react to produce theaflavin.
The tannase can be applied using a variety of art-known techniques. The present inventors prefer to dissolve tannase in water, spray the solution onto the dhool and leave the mixture to react for a suitable time at a suitable temperature. The tannase is applied to the dhool after an initial maceration (for example, a first CTC cut) in a fine spray followed by subsequent cutting (for example, a second or third CTC cut) to ensure adequate mixing. The dhool is preferably incubated under vacuum, or under anaerobic conditions for example in an atmosphere or nitrogen. This prevents fermentation occurring. It is preferable that complete degallation takes place before fermentation starts as this results in maximal theaflavin levels in the subsequent fermentation, which in turn leads to optimal colour generation.
The present inventors postulated that one might increase the efficiency by which certain exogenous compounds can access certain endogenous compounds of solid state tea by using a vacuum to draw the exogenous molecules into the macerated tea leaves and into contact with the compounds therein. Vacuum infiltration per se is known. However it has been used to force substances between cells rather than into cells. And those substances have tended to have small molecular weights.
The present inventors have however developed a method for bringing certain exogenous compounds into contact with endogenous compounds found in tea that involves vacuum infiltrating macerated tea leaves with those exogenous compounds and applied it to modify certain properties of tea and tea based beverages. The extent to which this method allows even large molecules such as enzymes to access endogenous tea compounds and modify certain properties of tea has been truly surprising. For example, an infusion of tannase pre-treated tea has been found to have more than double the total theaflavin content of a control and a six fold increase in TF1.
Vacuum infiltration is a technique that is often used in the preparation of protoplasts from plant tissue, albeit to introduce substances between rather than into cell walls. Cut leaf tissue is incubated in a solution containing tannase. The suspension is then placed under vacuum and air is drawn from the intracellular spaces of the leaf particles, the enzyme solution is drawn in to replace it. The inventors have found that a vacuum less than 100 mbar is suitable for this.
The major constraint when applying this method to tea dhool is achieving access within the cells. Another major problem is that large volumes of water can seriously affect the quality of tea, by reducing oxygen uptake during fermentation. The results described in the Examples indicate that vacuum infiltration is a useful tool for introducing enzymes, for example tannase, into solid state dhool. When fermented, tannase treated dhool gives rise to black tea with high levels of theaflavin and no gallated species. This enables one to produce a range of novel teas. Vacuum assisted tannase treatment is much more effecgive in removing gallated species and enhacing additional TF formation than the equivalent treatment under ambient temperature and pressure. The vacuum allows the enzyme to penetrate the tissue and remove gallated species prior to fermentation, they key feature of tannase driven theaflavin enhancement, compared to simply applying the enzyme onto fermenting dhool and mixing in by hand.
If at all possible the conditions should be adjusted to prevent any fermentation prior to or during tannase treatment. This can be achieved by using a stronger vacuum pump, holding the dhool under N2 sparge, or shortening the tannase treatment.
Tannase can be applied to the macerated tea by a simple dosing. However, spraying the tannase in a fine mist is preferably as it aids infiltration.
Suitable conditions can be readily determined by experiment. Good results have been obtained with KIKKOMAN""s tannase (KIKKOMAN is a trade mark of Kikkoman Corporation, Tokyo, Japan) in an amount of 1-100 mg/kg dhool, preferably 10-80 mg/kg dhool but more preferably 40-80 mg/kg dhool. Note: KIKKOMAN""s tannase has 50,000 tannase activity units/gram.
Fermentation is preferably carried out at a pH in the range of 4.0 to 5.5. The fermentation temperature is preferably in the range 15 to 40xc2x0 C. Fermentation is preferably carried out for a time in the range 30 to 150 minutes, more preferably 105 to 120 minutes. However in a second departure from traditional black tea manufacture hydrogen peroxide is added, after a time that is sufficient to generate gallic acid and theaflavin during the fermentation step, to activate (or at least greatly enhance the activity of) endogenous peroxidase.
Tea is known to contain natural peroxidase at high levels. It is also known that natural peroxidase can be activated (or have its activity enhanced) through the addition of hydrogen peroxide in a slurry system. J. Sci. Food Agric. vol. 32, p 920-932 (Dix., 1981) discloses such a system and process. The article mentions that peroxidase can oxidase tea polyphenols to form theaflavins and also thearubigens which may be similar and different to those produced under xe2x80x9cnormalxe2x80x9d fermentations. However it does not offer any detailed understanding as to the chemistry at work.
The present inventors have found that the endogenous peroxidases have the potential to oxidise catechins to theaflavins and thearubigens, convert theaflavins to thearubigens and, unlike endogenous polyphenol oxidase, readily oxidise gallic acid. The combination of these reactions generates significant amounts of coloured compounds that are soluble in cold water. The chemistry involved here is represented in FIG. 2.
The hydrogen peroxide is added in an amount that is sufficient to activate endogenous peroxidases and oxidise gallic acid liberated by the tannase treatment. One skilled in the art can determine that by experiment. However the present inventors prefer to use between 100 and 200 ml of 2.0 to 2.5% hydrogen peroxide per kg dhool, but preferably 160 ml of 2.0% hydrogen peroxide per kg dhool. Under normal conditions of tea manufacture peroxidase is largely inactive, due to the low endogenous levels of hydrogen peroxide and high activities of catalase. Measurements have shown that all added hydrogen peroxide is consumed during the process, with none remaining in the final made tea. In contrast to the findings disclosed in the aforementioned U.S. Pat. No. 4,051,264 the present inventors have found that the combination of tannase treatment and subsequent activation of peroxide is critical for the manufacture of a product that gives good colour and acceptable taste. Product that was only tannase treated had a xe2x80x9csourxe2x80x9d or xe2x80x9cmetallicxe2x80x9d note.
As one would expect, the colour and taste profile of a beverage made from the cold water infusing leaf tea of the present invention depends to a large extent on the source and quality of the raw material i.e. tea leaves. The present inventors have found that standard raw material, two leaves and a bud delivered to the factory within excess of 1100 shoots per kilogram green leaf tea, can be processed according to the method of the invention to give very good colour and taste. However during efforts to improve the colour and taste even further the inventors surprisingly found significant improvement to both can be achieved by using more mature leaf.
Tea is generally harvested as two leaves and a bud on a 17 day cycle to optimise quality and yield. Extending the cycle means the leaves will be more mature and their chemical composition will be a little different. One then needs to pluck larger portions of tea plant to account for the extra growth. Such a harvesting strategy increases the yield per hectare of tea and thus improves productivity but the harvested plant material tends to have longer stems and a higher stem to leaf ratio.
Black tea made from mature tea leaf material tends to be less coloured and thinner after infusion than black tea made from tea portions of two leaves and a bud that are harvested on a 17 day cycle. However, the present inventors surprisingly found that when mature leaf tea, i.e. tea leaves harvested on a 30 to 50 day cycle in portions of 3 to 5, but preferably 4 leaves and a bud, is used there is a significant increase in cold water infusion performance and thus an improvement in colour and taste.
While not wanting to be bound by theory, it is thought that mature leaf material (including stalk) contains higher levels of peroxidase than standard leaf and this peroxidase provides an important role in the maturation cascade. This means more peroxidase is available when hydrogen peroxide is added when carrying out the method of the invention. Consequently more colour is generated by an enhanced peroxide/peroxidase oxidation system.